Nickel and Iron Sulfide Nanoparticles from Thiobiurets - The Journal of

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Nickel and Iron Sulfide Nanoparticles from Thiobiurets Ahmed Lutfi Abdelhady,† Mohammad A. Malik, Paul O’Brien,* and Floriana Tuna School of Chemistry and School of Materials, The University of Manchester, Manchester, U.K. ABSTRACT: The nickel(II) and iron(III) complexes of 1,1,5,5-tetraiso-propyl-2-thiobiuret were used as single-source precursors for the synthesis of nickel and iron sulfide nanoparticles, respectively, by thermolysis in hot oleylamine, octadecene, or dodecanethiol. Several combinations of different injection solvents and capping agents were used in the reaction mixture. The shape and the phase of the material were controlled by changing the injection solvent, capping agent, growth temperature, or precursor concentration. The thermolysis of the nickel precursor gave Ni3S4 in all cases except when precursor solution in oleylamine was injected into hot octadecene which produced NiS nanoparticles. Ni3S4 was obtained in different morphologies (wires, rods, spheres, and triangles) depending upon the reaction conditions, whereas NiS was obtained as wires only. The thermolysis of the iron complex in oleylamine/oleylamine produced Fe7S8 nanoparticles with different morphologies (spherical, rods, or plates), again depending on the growth temperature and precursor concentration. Combinations of different injection solvents and capping agents produced, in most cases, amorphous material.

1. INTRODUCTION Transition metal chalcogenide nanocrystals have been extensively explored because of their interesting physical and chemical properties.1 5 These properties are shape-dependent, and hence good control is required from the synthetic methods used.6 8 Zerodimensional (OD) or quantum dots have been widely studied and can be synthesized by different chemical methods.9 13 Various nanoscale devices have been fabricated based on quantum dots, including light-emitting diodes,14 sensors,15 optical detectors,16 quantum dot lasers,17 and memory units.18 One-dimensional (1D) nanostructures such as wires, rods, and tubes are considered to be an enhanced system to explore the relation between electrical or thermal transport or mechanical properties and the size confinement.6,19 These structures are involved in the fabrication of electronic, optoelectronic, and electrochemical devices.19 Recently, we reported the use of copper(II) thiobiuret as a singlesource precursor for the synthesis of good quality hexagonal copper sulfide nanodisks.20 In this paper we report the use of nickel(II) and iron(III) thiobiuret complexes as single-source precursors to synthesize nickel sulfide and iron sulfide nanoparticles. Nickel sulfide is known to have various stoichiometries: Ni3S2, Ni3+xS2, Ni4S3+x, Ni6S5, Ni7S6, Ni9S8, Ni3S4, and NiS.21 25 These differing stoichiometries make nickel sulfide attractive and interesting but complicated to study. Nickel sulfide has a wide range of applications, such as in: electrodes, battery materials, hydrogenation catalysts, and transformation tougheners for complex ceramics.26 29 Various methods have been used to synthesize nickel sulfide nanostructures. A solvothermal method was used to synthesize urchin-like NiS.30,31 NiS hollow nanospheres were synthesized via γ-irradiation.32 A sonochemical method33 and a microemulsion system34 produced NiS nanoparticles. Korgel et al.3 synthesized the Ni3S4 nanocrystal by the thermal decomposition of NiCl2 and elemental sulfur in oleylamine. The same group reported the solventless thermolysis of r 2011 American Chemical Society

a nickel alkylthiolate molecular precursor, producing nanorods and triangular nanoprisms of NiS.35 Thermal decomposition of singlesource precursors such as alkyl xanthates,4 mercaptobenzothiazole,1(tetramethylethylenediamine)Ni(SCOC6H5)2,36 and polysulfide [Ni(N-methylimidazole)6]S837 in a hot coordinating solvent formed a mixture of rods and spheres of NiS, ellipsoidal NiS nanoparticles, alpha or beta NiS nanocrystals, and NiS2 or Ni1 xS nanocrystals, respectively. Iron sulfide also exists in different forms, including pyrite (cubic-FeS2), marcasite (calcium chloride structure-FeS2), troilite (FeS), mackinawite (Fe1+xS), pyrrhotite (Fe1 xS), smythite (hexagonal-Fe3S4), and greigite (cubic spinel-Fe3S4).38 40 Pyrrhotite (Fe1 xS, x = 0 0.125) is a nonstoichiometric form of numerous known superstructures based on the different arrangement of the iron vacancies.40 44 Fe7S8 (x = 0.125) exists in a monoclinic, trigonal, or hexagonal phase.41 44 Monoclinic and trigonal Fe7S8 are ferrimagnetic and stable below 254 and 262 °C, respectively, while the hexagonal phase is antiferromagnetic and stable below 315 °C.41 44 Fe9S10, Fe10S11, and Fe11S12 are antiferromagnetic, stable below 209 °C, and generally known as hexagonal pyrrhotite.41 44 Most of the materials reported as photovoltaic use either toxic or not very abundant elements such as cadmium, lead, or indium, etc., which means that these materials cannot contribute significantly to a future energy supply. Cheap nontoxic and abundant materials are much more promising and could make a significant impact even with lower efficiencies. Recent estimates of the annual electricity potential as well as material extraction costs and environmental friendliness led to the identification of materials that could be used in Received: August 16, 2011 Revised: November 28, 2011 Published: December 12, 2011 2253

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The Journal of Physical Chemistry C photovoltaic applications on a large scale.45 The most promising materials include iron and copper sulfides. Very recently, Law et al.46 reported the synthesis of colloidal FeS2 nanocrystal inks for thin film photovoltaics. Hydrothermal and solvothermal routes have been widely investigated for the synthesis of iron sulfide in different phases and shapes, including microrods of Fe3S4 and FeS2,47 Fe3S4 flower-like microspheres,48 FeS2 crystallites with cubic and octahedral shapes,49 Fe3S4 nanosheets and nanoparticles in a mixed solvent of ethylene glycol and H2O,50 Fe0.985S necklace-like chains with no surfactant added,51 and cubic FeS2 via a single-source approach using iron(III) diethyldithiophosphate52 or iron diethyldithiocarbamate.53 Fe7S8 nanowire arrays have been prepared by electrodeposition with anodic aluminum oxide (AAO) films. 54 Distinctive nanostructures with dumbbell-like shape formed of hexagonal plates and nanowires with compositions of Fe9S8 and Fe7S8, respectively, have been synthesized by a chemical evaporation method.55 Various iron sulfide forms (Fe1 xS,37,56 Fe7S8,57 59 and Fe3S457 60) have been prepared by the thermal decomposition of different singlesource precursors, such as alkylthiolate,56 dithiocarbamates,57,60 bis(tetra-n-butylammonium)tetrakis[benezenethiolato-μ3sulfidoiron],59 and [Fe(N-methylimidazole)6]S8.37 We have previously reported the use of thiobiuret complexes61 for the deposition of Ni9S862 and FeS,63 thin films by aerosolassisted chemical vapor deposition. Herein, we report the colloidal thermolysis of these complexes to synthesize Ni3S4, NiS, and Fe7S8 nanoparticles with different morphologies.

2. EXPERIMENTAL SECTION All preparations were performed under an inert atmosphere of dry nitrogen using standard Schlenk techniques. Dodecanethiol (DDT) was purchased from Fluka. All other reagents were purchased from Sigma-Aldrich chemical company and used as received. Solvents were distilled prior to use. 2.1. Synthesis of [Ni(SON(CNiPr2)2)2] (1). As described in the literature,61 a solution of di-iso-propylcarbamoyl chloride (1.0 g, 6 mmol) and sodium thiocyanate (0.49 g, 6 mmol) in acetonitrile (25 mL) was heated to reflux with continuous stirring for 1 h, during which time a fine precipitate of sodium chloride formed. To the cooled reaction mixture was added di-iso-propylamine (1.49 mL, 12 mmol) followed by stirring for 30 min, and addition of nickel(II) acetate (0.76 g, 3 mmol) gave the product as violet crystals. 2.2. Synthesis of [Fe(SON(CNiPr2)2)3] (2). The same method as for 1 but using iron(III) nitrate (0.82 g, 2 mmol). The product was obtained as a red powder.61 2.3. Synthesis of Nanoparticles. All the nanoparticles were synthesized by thermal decomposition of the single-source precursor. Thermolysis experiments were carried out under different conditions: using three different concentrations (5, 10, and 20 mM) at 200 °C, three different temperatures (200, 240, and 280 °C) at a concentration of 5 mM, and different capping agents at a fixed temperature and a fixed concentration. In a typical experiment, oleylamine (OLA) (15 mL) was degassed under reduced pressure at 100 °C for 30 min and then heated to the desired temperature under nitrogen. The required amount of the precursor [Ni(SON(CNiPr2)2)2] or [Fe(SON(CNiPr2)2)3] was dispersed in OLA (5 mL) and injected into the hot OLA. The reaction was maintained for 1 h. The solution formed was cooled to approximately 70 °C. After cooling the reaction mixture, an excess of methanol was added, and the solid was

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Figure 1. p-XRD patterns of nickel sulfide nanoparticles using (a) 5 mM, (b) 10 mM, and (c) 20 mM precursor solution at 200 °C. Red and blue lines represent the Ni3S4 (Polydymite) (ICDD card No. 043-1469) and Ni9S8 (ICDD card No. 022-1193) peaks, respectively.

isolated by centrifugation. The solid was washed several times with methanol and then redispersed in toluene. Any insoluble material was removed by centrifugation. 2.4. Characterization of Nanoparticles. X-ray diffraction studies were performed on a Bruker AXS D8 diffractometer using Cu Kα radiation. The samples were mounted flat and scanned between 10 and 80° in a step size of 0.05 with a count rate of 9 s. Transmission electron microscopy (TEM), highresolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) analysis were performed using a Tecnai F30 FEG TEM instrument, operating at 300 kV, with all samples deposited over carbon-coated copper grids. Magnetic measurements were performed using a Quantum Design MPMS XL SQUID magnetometer equipped with a 7 T magnet. Variable field magnetization data were collected at temperatures of 5 and 300 K, by cycling the magnetic field between 40 and 40 kOe. Variable-temperature zero-field (ZF) and field-cooled (FC) magnetization data were collected in the temperature range 5 300 K, at an applied magnetic field of 100 Oe. Caution. Note that nickel sulfides are carcinogens.

3. RESULTS AND DISCUSSION 3.1. Nickel Sulfide Nanoparticles. Nanoparticles of nickel sulfide were obtained at different thermolysis temperatures and concentrations. The p-XRD patterns of the nickel sulfide nanoparticles obtained from all experiments correspond to cubic Ni3S4 (Polydymite) (ICDD card No. 043-1469) with minor impurities of orthorhombic Ni9S8 (Godlevskite) (ICDD card No. 022-1193) at higher precursor concentrations (10 and 20 mM), as shown in Figure 1. TEM images of the nanoparticles grown at different temperatures and concentrations showed remarkable changes in the shape of crystallites (Figure 2). Nanowires (g250 nm) with a width between 5 and 10 nm were obtained at 200 °C (Figure 2(a)) when a 5 mM solution of the precursor in oleylamine was used. Increasing the concentration to 10 mM led to the formation of nanorods 20 65 nm in length and with an average width of 8.6 ( 1.7 nm (Figure 2(b)). On using the higher concentration (20 mM), a mixture of spherical nanoparticles and nanorods was produced (Figure 2(c)). The length of the nanorods varied between 24 and 66 nm, whereas the average 2254

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Figure 2. TEM images of nickel sulfide synthesized using (a) 5 mM, (b) 10 mM, and (c) 20 mM precursor solution at 200 °C and (d and e) using 5 mM at 240 and 280 °C, respectively.

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Figure 4. (a) HRTEM of spherical nanoparticles, (b and c) HRTEM of nanowires, and (d f) FFT of (a c), respectively.

Figure 5. p-XDR patterns of nickel sulfide using different injection solvents/capping agents: (a) OLA/OLA, (b) DDT/OLA, (c) ODE/ OLA, (d) OLA/DDT, (e) ODE/DDT, and (f) OLA/ODE. Red and blue lines represent the Ni3S4 (Polydymite) (ICDD card No. 043-1469) and NiS (ICDD card No. 002-1280). Figure 3. TEM images of nickel sulfide synthesized using 20 mM precursor solution at 200 °C after (a) 5 min, (b) 30 min, and (c) 1 h. (d f) Histograms of spherical particles in (a c), respectively, and (g i) histograms of rods in (a c), respectively.

diameter of nanoparticles was 14.2 ( 1.5 nm. Increasing the growth temperature had a similar effect on the size and shape of nanoparticles. The lengths of nanowires decreased from 250 nm (200 °C) to 140 190 nm (240 °C), whereas the width was almost unaffected (Figure 2(d)). A further increase in thermolysis temperature (280 °C) resulted in the formation of mainly spherical nanoparticles with an average diameter of 13.1 ( 1.2 nm (Figure 2(e)). From these results, it can be suggested that at lower growth temperature or lower precursor concentration fewer nuclei are formed which favors the growth of elongated nanocrystals.8,19 By increasing the concentration or temperature, more nuclei are formed, due to the presence of more material or increasing the reactivity of the precursor, leaving a lower monomer concentration in solution which directs the nanocrystal growth toward the lowest chemical potential environment and results in the formation of spherical nanocrystals.8,19

At the highest concentration (20 mM) of precursor at 200 °C, a mixture of nanorods and spherical nanoparticles was obtained (Figure 2(c)). The reaction mixture was analyzed at different time intervals to observe the effect of time on the shape of nanoparticles (Figure 3(a, b, and c)). Figure 3(a, b, and c) shows that both the nanorods and the spherical nanoparticles are formed from the beginning of the reaction. Neither of these two morphologies grew at the expense of the other; the rods to spherical nanoparticles ratio roughly remains the same. The nanorod length increased by time from the range of 11 30 nm after 5 min to 15 44 nm and 24 66 nm after 30 and 60 min, respectively. The average diameter of the spherical nanoparticles was 9.4 ( 1.2, 13.5 ( 1.5, and 14.2 ( 1.5 nm after 5, 30, and 60 min, respectively. The defocusing in size distribution could be assigned to the Ostwald ripening effect. By the time the monomer is consumed and its concentration depletes, the critical radius for the formation of stable particles increases, and larger particles grow on the expense of smaller ones.8 The d-spacing obtained for both spherical nanoparticles and the nanorods from HRTEM and Fast Fourier Transform (FFT) (Figure 4) is about 2.80 Å, which corresponds to the (113) planes of the cubic Ni3S4 (ICDD card No. 043-1469). An extra pair of 2255

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Figure 8. p-XRD patterns of iron sulfide nanoparticles at (a) 200 °C, (b) 240 °C, and (c) 280 °C using 5 mM solution. Figure 6. TEM images of nickel sulfide using different injection solvents/capping agents: (a) OLA/OLA, (b) ODE/OLA, (c) OLA/ ODE, (d) DDT/OLA, (e) OLA/DDT, and (f) ODE/DDT.

Figure 7. (a) HRTEM of a triangular nanoprism and (b) FFT of (a).

Figure 9. TEM images of iron sulfide synthesized using (a) 5 mM, (b) 10 mM, and (c) 20 mM precursor solution at 200 °C and (d and e) using 5 mM at 240 and 280 °C, respectively.

spots was observed in the FFT of both spherical nanoparticles and the nanorods that could be indexed to the (004) planes. The two extra spots in the FFT of the spherical nanoparticles originate from the top right part of the crystal which could be due to growth on the (004) planes or due to stacking faults. The source of these extra spots in FFT of the nanorods is a neighboring crystal growing on the (004) plane. Another HRTEM image (Figure 4(c)) of a nanowire and its FFT (Figure 4(f)) show d-spacing corresponding to the (113) planes and double spacing of the (004) planes. The effect of other surfactants or capping agents was investigated using 5 mM solution of the precursor at 240 °C. p-XRD patterns of the as-obtained nanoparticles are shown in Figure 5. Replacing OLA from the injection solution with the noncoordinating solvent octadecene (ODE) produced Ni3S4 nanorods with average length and width of 30 and 7 nm, respectively (Figure 6(b)), instead of longer nanowires as observed for OLA as injection solvent and capping agent (Figure 6(a)). When an OLA solution of the precursor was injected into hot ODE, highly aggregated NiS (ICDD card No. 002-1280) nanowires were obtained (Figure 6(c)). The change in the phase of the produced particles and the aggregation of the particles could be due to the noncoordinating nature of ODE. DDT showed a great effect on the shape of the nanoparticles. Replacing OLA from the injection solution with DDT produced Ni3S4 triangular-based nanostructures (nanoprisms or tetrahedrons) with a few nanorods as well (Figure 6(d)). When an OLA solution of the precursor was injected into hot DDT, all nanorods disappeared, and only

the triangular-based nanostructures could be seen in TEM (Figure 6(e)). The HRTEM image of the triangular-based nanostructures may suggest a tetrahedron structure as the contrast decreases from the apex to the base indicating a decrease in thickness (Figure 7).64 Nanorods were again obtained by dissolving the precursor into ODE and then injecting the solution into hot DDT (Figure 6(f)). According to Korgel et al.,35 not much information is available on the alkanethiol adsorption on nickel surfaces. However, it is well-known that some surfactants can bind to different facets of the nanoparticle with different strength, hence altering the growth rate of these different facets.65 Korgel et al.35 have also reported the synthesis of triangular nickel sulfide nanoparticles in the presence of DDT which is in agreement with our results. In previous reports, the phase of the produced nickel sulfide nanostructures was found to be highly dependent on the reaction temperature, time, and solvent.66,67 In our results, there was no significant change in the phase of the nickel sulfide nanomaterials synthesized at different growth temperatures. We observed phase change only at higher concentrations of the precursor or on using large amounts of ODE. 3.2. Iron Sulfide Nanoparticles. Nanoparticles obtained from all different experiments were analyzed by p-XRD and TEM. All reactions in OLA/OLA produced Fe7S8 (Pyrrhotite-4 M ICDD card No. 029-0723) (Figure 8) except at high concentration (20 mM) which gave only amorphous material. TEM images of the nanomaterials grown at different temperatures and concentrations showed remarkable changes 2256

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Figure 10. TEM images of iron sulfide synthesized using 5 mM precursor solution at 280 °C after (a) 5 min, (b) 30 min, and (c) 1 h.

Figure 12. TEM images of iron sulfide using different injection solvents/capping agents: (a) ODE/OLA, (b) DDT/OLA, (c) OLA/ ODE, and (d) OLA/DDT.

Figure 11. (a and c) HRTEM of spherical Fe7S8 nanoparticles, (b) FFT of (a), (d) HRTEM of a polyhedral Fe7S8 nanoparticle, (e) HRTEM of hexagonal Fe7S8 nanoparticle, and (f) SAED of Fe7S8 obtained from a mixture of hexagonal nanoplates and nanorods.

in the morphology of crystallites (Figure 9). At 5 mM solution of the precursor and 200 °C growth temperature, good quality nanoparticles with an average diameter of 5.1 ( 1.0 nm were obtained (Figure 9(a)). Increasing the concentration to 10 mM allowed the nanoparticles to grow bigger and resulted in an average diameter of 6.1 ( 0.9 nm (Figure 9(b)), whereas at higher concentration of 20 mM only amorphous material is observed (Figure 9(c)). A significant change in the morphology of obtained nanomaterials was observed on increasing the growth temperature from 200 to 240 or 280 °C. At 240 °C, irregular nanocrystals were formed (Figure 9(d)). At the higher temperature (280 °C), hexagonal nanoplates and nanorods were obtained (Figure 9(e)). Both structures had a wide range of size distributions as the width of the hexagonal

nanoplates varied from 20 to 120 nm, and the nanorod lengths were between 37 and 95 nm. The disappearance of the irregular structures at high temperature (280 °C) is in agreement with previous results.58 The spherical morphology is known to be thermodynamically the most stable, whereas the hexagonal and rod structures are comparatively less stable. 68 Therefore, it could be suggested that the thermolysis reaction at higher temperature (280 °C) is kinetically controlled. Figure 10 shows a study of the effect of reaction time at 280 °C and 5 mM concentration. Samples analyzed by TEM after 5 min of the start of reaction showed hexagonal nanoplates and nanorods (Figure 10(a)). After 30 min, a mixture of well-defined hexagonal nanoplates with width range between 20 and 60 nm and nanorods with length range of 25 to 60 nm was obtained (Figure 10(b)). After 1 h these structures grew bigger and reached 120 nm for the hexagonal nanoplate width and 95 nm for the nanorod length (Figure 10(c)). Lattice fringes observed in the HRTEM images indicate the high crystallinity of the nanoparticles. The spherical nanoparticles showed a d-spacing of 2.08 and 2.67 Å corresponding to the (322) and (004) planes, respectively (Figure 11(a,c)). HRTEM revealed that some of the spherical nanoparticles are actually polyhedron (Figure 11(d)). The d-spacing calculated from the hexagonal nanoparticles was found to be 3.03 Å corresponding to the ( 122) plane (Figure 11(e)). The selected area diffraction pattern confirmed the formation of Fe7S8 (Pyrrhotite-4 M ICDD card No. 029-0723) (Figure 11(f)). The effect of other surfactants or capping agents was investigated using 5 mM solution of the precursor at 200 °C. Injecting an ODE solution of the precursor into hot OLA or vice versa produced nanowires of varying length and orientation (Figure 12(a, c)), whereas injecting a DDT solution of the precursor into hot OLA produced monodispersed nanowires 2257

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Figure 13. (a) p-XRD pattern of Fe7S8 from OLA/DDT. (b) HRTEM of Fe7S8 from OLA/DDT and (c) FFT of (b).

Figure 14. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves for (a) nickel sulfide nanowires (>250 nm) and (b) iron sulfide nanocrystals (5.1 ( 1.0 nm) obtained from a 5 mM solution at 200 °C. Insets are magnetic hysteresis loops at 5 and 300 K.

with length up to 90 nm and average diameter of 1.5 nm (Figure 12(b)). Injection of OLA solution of the precursor into hot DDT produced a cluster of nanowires (Figure 12(d)). The p-XRD pattern of the wire cluster showed the growth of Fe7S8 (Figure 13(a)). A d-spacing corresponding to the ( 122) and (004) planes was calculated from both HRTEM and FFT images (Figure 13(b,c)). 3.3. Magnetic Properties. The magnetic properties for Ni3S4 nanowires (>250 nm) and Fe7S8 (5.1 ( 1.0 nm) nanocrystals prepared using a 5  10 3 M solution at 200 °C are shown in Figure 14(a) and 14(b), respectively. The Ni3S4 sample is paramagnetic at room temperature; a ferrimagnetic transition from antiferromagnetic to ferromagnetic occurs at ca. 15 K, and the sample undergoes a magnetic transition below 15 K with wide hysteresis (ca. 1000 Oe coercive field at 5 K). There is also remnant magnetization at 5 K of ca. 1.5 emu/g. The Fe7S8 sample shows the magnetic behavior of the monoclinic pyrrhotite, with superparamagentic behavior at room temperature, which is usually observed when the size of the particles is small, and a very characteristic transition at ca. 25 30 K. The hysteresis curve at 5 K is characterized by a coercive field of 270 Oe and a remnant magnetization of ca. 0.45 emu/g.

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Figure 15. Graphical representation of iron sulfide nanoparticles showing different phases as reported in the literature by the thermolysis of different single-source precursors in OLA at different temperatures, compared to the relative thermodynamic stabilities of the various phases of iron sulfide after Vaughan.38 [Fe(S2CNEtiPr)3] (green),60 [Fe(S2 CN(Hex)2)3] (blue),60 [Fe(S2CNEtMe)3] (violet),60 [Fe(SON(CNi Pr2)2)3] (orange) this work, [Fe(S2CNEt2)3] (gray57 and black58), [Fe(S2CNEt2)2(phenanthroline)] (pink57 and light blue58), and [Fe(Nmethylimidazole)6]S8 (yellow).37

4. CONCLUSIONS Colloidal thermolysis of nickel(II) and iron(III) complexes of 1,1,5,5-tetra-iso-propyl-2-thiobiurets in OLA produced Ni3S4 and Fe7S8, respectively. Ni3S4 was obtained from all reactions except when precursor solution in oleylamine was injected into hot octadecene which produced NiS nanoparticles. Fe7S8 was produced from the thermolysis of the iron complex in OLA/ OLA or OLA/DDT only. All other combinations of injection solvent/capping agent gave amorphous material. The morphology of the obtained nanoparticles was highly dependent on the reaction parameters (growth temperature, precursor concentration, and injection solvent/capping agent mixture). Figure 15 shows a plot developed by our group63 and by Vaughan and Lennie,38 representing the relative stabilities of the various phases of iron sulfide nanoparticles obtained from the thermolysis of different single-source precursors in OLA at different temperatures. The height of the pyramid on the negative z-axis represents the free energy of formation of each phase. The solid line represents the thermodynamic stability and connects the stable phases FeS (troilite) and FeS2 (pyrite).38 Our work produced Fe7S8 as the only phase (shown in orange) at all temperatures which suggests that the reaction product was kinetically controlled. ’ AUTHOR INFORMATION Corresponding Author

*Fax: 44 161 275 4598. Tel.: 44 161 275 4653. E-mail: paul. [email protected]. Present Addresses †

Faculty of Science, Mansoura University, Mansoura, Egypt.

’ ACKNOWLEDGMENT A.L.A. gratefully acknowledges financial support from the Egyptian Cultural Affairs and Missions Sector. The authors also thank EPSRC, UK, for the grants to POB that have made this research possible. POB wrote this manuscript while a Visiting 2258

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The Journal of Physical Chemistry C Fellow at IAS University of Durham. He would like to thank the University for the Fellowship and Collingwood College and its Fellows for being gracious hosts.

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